The reproductive rate differs considerably in different species of animals. Some, such as man or a tsetse fly, produce only one young at a birth, while at the other extreme various fish and marine invertebrates lay well over a million eggs at a spawning. The variations are quite large even within one class of animals. In birds, for instance, the average clutch - size of different species varies between 1 and 15. Even within the same species the variations may be far from negligible. Thus the average clutch - size of different populations of the Great Tit Parus major varies from 3 to 12. (MacRoberts, B. R. and M. H. MacRoberts. 1972)
In the past, variations in the reproductive rate have usually been explained as due to differences in the mortality rate. Migratory birds have larger clutches than resident species because they have to undergo the additional danger of a long journey. Elephants have only one young at a birth because they are long-lived and, if they had more, they would quickly overpopulate Africa. Flatfish lay vastly more eggs than skates to compensate for the vastly heavier mortality among their fry. So runs the argument, which appears in a variety of explicit and implicit forms through the zoological literature.
The fact is undoubted: those species with a lower reproductive rate have a lower mortality rate; but the explanation must be wrong. The reproductive rate is a product of evolution, and if the theory of natural selection be accepted, then the number of eggs laid by each species should be that which results in the maximum number of surviving offspring; and those genotypes with a higher egg-number than the normal should be eliminated by selection. Is this statement true? If it is true, what is the selective disadvantage of genotypes with........
